The present invention relates in general to optical signal processing (e.g., switching) systems and components therefor, and is particularly directed to an arrangement for correcting for the inherent field curvature of a multi-optical signal focusing element, such as a concave mirror, upon the planar surface of an optical signal control (e.g., switching) device, such as a micro electro-mechanical switch (MEMS) or a liquid crystal array. Correction for this field curvature is accomplished by defining an auxiliary focal plane as a xe2x80x98best fitxe2x80x99 approximation of the curved focal plane of the focusing element surface, and positioning the MEMS such that its planar, optical signal-receiving surface intersects the auxiliary focal plane.
In addition, the transmission paths of the optical signals focused by the focusing element are modified, as by means of a transmissive optical wedge, so as to make the auxiliary focal plane effectively coplanar with the optical signal-receiving surface of the MEMS. Such correction is useful in free space devices, such as a configurable optical add/drop multiplexer (COADM), a dynamic gain equalizer (DGE), wavelength blocker or dynamic channel equalizer, a free space switch, or a wavelength switch, where spatially separated beams must be approximately focused on a planar device by an optical element having a curved focal field. It may be noted that two conditions are caused by defocusing at the reflection plane. First, the principal ray is not perpendicular to the processing plane and introduces losses by introducing positional offsets and thus poorly coupling into an output optical fiber. Second, if the beam is defocused at the processing plane, a broader beam (not the focus point) is reflected, so that a broader beam is poorly coupled into the output fiber.
As described in the above-referenced ""270 application, wavelength division multiplexed (WDM)-based optical communication systems often employ configurable optical add/drop multiplexers (COADMs) to selectively add and/or drop one or more channels (wavelengths) from a composite optical signal stream, as may be transported over a multichannel optical signal waveguide. As a non-limiting example, FIG. 1 diagrammatically illustrates the general architecture of a COADM of the type disclosed in the ""270 application. A COADM is an example of a limited multi-wavelength switch; of course, multiple wavelength switches with larger numbers of channels and greater switching flexibility are also possible.
As shown therein the COADM comprises first and second optical circulators 10 and 20, respectively coupled to first and second optical waveguides 31 and 32, and used to separate in/out and add/drop optical signals. In order to interface external optical channels with respect to the COADM, the first circulator 10 has an input or IN port 11, and an output or OUT EXPRESS port 12. A third port 13 of the circulator 10 is coupled to the first optical waveguide 31. Similarly, the second circulator 20 has an input or ADD port 21, and an output or OUT DROP port 22. A third port 23 of the circulator 20 is coupled to the second optical waveguide 32.
A focusing lens 35 is coupled to focus optical signals transported by waveguides 31 and 32 at the focal plane 45 of the lens 35 to be coincident with a point N of the curved focal field LP of the focusing element 40, such as a reflective surface of revolution, for example, a spherical reflector. Beams 41 and 42 launched from the waveguides 31 and 32 parallel to the axis of the lens 35 (which does not have to be parallel to axis of reflector 40) emerge from the lens 35 to intersect at a focal point N, where the curved focal field LP of the reflector 40 and the focal plane 45 of lens 35 are substantially coincident. Passing through focal point N, beams 41 and 42 define an opening angle xcex3.
These two beams are reflected from the spherical reflector 40 upon a dispersive element 50 such as a diffraction grating, which spatially disperses the respective optical frequency components of the two beams along different directions in accordance with their various wavelengths components xcex. The diffraction grating may be coplanar with, or it may intersect and be oriented at an angle with respect to the focal plane 45. The wavelengths dispersed by the diffraction grating are reflected by the spherical reflector 40, so as to be incident upon the control surface 65 of a beam modifier 60, such as, but not limited to a digitally controlled micro electro-mechanical switch (MEMS) array, such as one containing a one- or two-dimensional Mxc3x97N distribution micro-mirrors, located in focal plane 45, each micro-mirror being selectively positionable in one of two bistable orientations.
For purposes of a reduced complexity illustration, the MEMS array 60 will be assumed to have a 2xc3x972 bypass configuration associated with the four external ports of the two circulators 10 and 20. Also, in the present example, a respective beam of light includes only first and second wavelengths xcex1 and xcex2, although more than two wavelengths are typically used in practice. In a first operational state, the MEMS array 60 is operative to cause an xe2x80x98expressxe2x80x99 signal-associated wavelength, contained in the optical signal that is launched into the IN port 11 of the circulator 10, to propagate to the EXPRESS port 12 of the same circulator 10, and a xe2x80x98dropxe2x80x99 signal-associated wavelength signal, that is launched into port 11 of the circulator 10, to propagate to port 22 of circulator 20 in a second operational state. Conversely, a wavelength supplied to port 21 of second circulator 20 propagates to port 22 of the second circulator 20 in the second mode of operation, but is not collected in the first mode of operation.
In operation, an input optical signal beam containing each of the two wavelengths xcex1 and xcex2 is launched into the port 11 of the first optical circulator 10 and is circulated thereby to port 12 and coupled to the optical waveguide 31. This multi-wavelength beam of light is transmitted through waveguide 31 and is directed therefrom upon the lens 35 in a direction that is substantially parallel to its optical axis. Lens 35 then directs the light beam to the spherical reflector 40 at Point A, from which the beam is reflected, so as to be incident on the diffraction grating 50 at point B, where it is spatially dispersed into two sub-beams of light respectively corresponding to the two wavelengths xcex1 and xcex2. Each of these sub-beams is incident upon points C1, C2 of the spherical reflector 40, and is reflected thereby, so as to be incident upon respective xe2x80x98microxe2x80x99 reflectors 61-D1 and 62-D2 of the MEMS array 60.
The reflector 61 may be controllably oriented in a first of its two bistable orientations, such that the sub-beam of light corresponding to the first dispersed wavelength xcex1, is reflected back along the same optical path to the lens 35, enters waveguide 31 again and propagates therethrough to port 13 of the circulator 10, where it is circulated to the OUT/EXPRESS port 12. The reflector 62, however, is oriented such that the sub-beam of light corresponding to the second dispersed wavelength xcex2 is reflected back along a different optical path. As a consequence, the dropped signal corresponding to the wavelength xcex2 is returned to the lens 35 via point E of the reflector 40, point F of the diffraction grating 50 and again to point G of the reflector 40, at an angle opposite to its input angle, so that it enters the waveguide 32 and propagates therethrough to the port 23 of the second circulator 20, wherein it is circulated to the OUT/DROP port 22.
Simultaneously with this operation, a second beam of light having wavelength xcex2 may be supplied as an ADDED input beam into the IN/ADD port 21 of the second optical circulator 20, so as to be circulated to port 23 and coupled to the optical waveguide 32. The second wavelength xcex2 is transmitted through optical waveguide 32 and directed upon lens 35 in a direction that is substantially parallel to its optical axis. Lens 35 then directs the second beam to point G of the spherical reflector 10, whereupon the second beam is reflected, so as to be incident on point F of the diffraction grating 50, from which it is directed, via the spherical reflector 40 at point E, to the reflector 62 of the MEMS array 60.
The reflector 62 is oriented in one of its two bistable orientations such that the second wavelength beam xcex2 is reflected back along a different optical path to the spherical reflector 40 at point C2, where it is directed to the diffraction grating 50 at point B. At the diffraction grating, the added optical signal corresponding to the second wavelength xcex2 is combined (multiplexed) with the express signal corresponding to the first wavelength xcex1. This multiplexed optical signal is then returned to the lens 35 via point A of the reflector 40, passes into the first optical waveguide 31 and is transported therethrough to port 13 of the first circulator 10, where it is circulated out from the OUT/EXPRESS port 12.
Now although such a COADM architecture is a very effective mechanism for providing precision control of the insertion and removal of selected wavelength components from a composite optical signal beam, it employs a focusing element, such as the spherical mirror 40 or a lens, having a curved focal field. As a result, different channels (wavelengths) are focused by this element at different points on a curved, rather than the planar focal surface FP. This curved focal field surface causes varying amounts of loss to be introduced into the optical channels that are diffracted onto the planar surface of the MEMS, or other planar optical signal control device.
For a further illustration of documentation describing multiplexed optical signal processing systems and components therefor, attention may be directed to the following publications: J. E. Ford, J. A. Walker, xe2x80x9cDynamic Spectral Power Equalization Using Micro-Opto-Mechanics,xe2x80x9d IEEE Photonics Technology Letters, Vol. 10, No 10, October 1998; J. E. Ford, V. A. Aksyuk, D. J. Bishop, J. A. Walker, xe2x80x9cWavelength Add-Drop Switching Using Tilting Micromirrors,xe2x80x9d Journal of Lightwave Technology, Vol. 17, No 5, May 1999; and N.A.
Riza, S. Yuan, xe2x80x9cReconfigurable Wavelength Add-Drop Filtering Based on a Banyan Network Topology and Ferroelectric Liquid Crystal Fiber-Optic Switches,xe2x80x9d Journal of Lightwave Technology, Vol. 17, No. 9, September 1999.
In accordance with the present invention, the above-described loss variation problem associated with the field curvature of a focusing element, such as a concave (spherical) mirror, of a configurable optical add/drop multiplexer of the type described above, is successfully addressed by defining a xe2x80x98best fitxe2x80x99 planar surface approximation of the curved focal plane of the focusing element, and locating the optical signal processing element (e.g., MEMS or liquid crystal array), so that its planar, optical signal-receiving surface coincides with the xe2x80x98best fitxe2x80x99 planar surface approximation. This is achieved by using a xe2x80x98field-flatteningxe2x80x99 element, such as a transmissive optical wedge, installed between the focusing element and the optical signal processing element.
The field-flattening optical wedge functions to modify the paths of the optical signals focused by the focusing element, so that it effectively tilts or rotates the xe2x80x98best fitxe2x80x99 planar surface approximation into coplanar coincidence with the optical signal-receiving surface of the optical signal processing element. With the curvilinear focal surface of the spherical mirror now being transformed into a focal plane, and that plane being coincident with the MEMS array plane, variation in loss (as minimized by the xe2x80x98best fitxe2x80x99 linear approximation of the focal plane) is effectively eliminated.